Georeferencing Tool
What is it?
Georeferencing is the process of adding geographic information to a map. The process involves identifying a set of ground control points (GCPs) with known coordinates and registering those point on the map using spatial editing process. AquaNET Georeferencing tool uses gdal_translate and gdal_warp tools in Python.Any image with geographic information can be displayed with GIS software (ArcGIS, QGIS etc.) or used to perform spatial analysis. AquaNET Georeferencing Tool allows users to georeference and image, and display as an image and/or download as a raster map.
How to use it:
The georererencing process involves four major steps:1. Load an image
2. Select ground control points on the image
3. Select the location (latitude/longitude) of each of the ground control points on the map.
4. Select transformation procedure and resampling method.
Loading and displaying an image
Click 'Load Image' button to load an image. Image can be in jpg or png format. Scroll the mouse wheel up to zoom in to a location. Scroll the mouse wheel down to zoom out. To reset the image view, click 'Reset Image' button.Select ground control points on the image
Double click on any image location to add a ground control point on the map. Once clicked, a number is displayed on the clicked location, and location of the point on the image is added to the table on the right. To delete a point, click on a row in the table containing the point to be deleted, and click 'Delete Row'. To move a point to a new location, click on a row in the table containing the point to move, click 'Move point' and double click to a new location.Register ground control points on background map
To register a ground control point, click on a row in the table containing the point to register, and click on the background map. Latitude and longitude of the clicked location is displayed. Once satisfied, click 'Add to table'. The latitude and longitude of the clicked location is added to the table. To move the point on the map to a new location, simply repeat the process.Select transformation procedure and resampling method
Select transformation procedeure from the drop-down list next to Method.MAGNET georeferencing tool provides four transformation methods that include polynomial and thin plate spline.
1. First order polynomial or affine transformation.
This is the commonly used method for georeferencing. This method shifts, scales and rotates image to match the ground control points with least-squares fitting algorithm. This method needs at least three ground control points.
2. Second and third order polynomial transformation.
Second and third order polynomial transformation bend or curve the image to fit the ground control points. They need at least 6 and 10 ground control points, respectively.
3. Thin plate spline transformation
Thin plate spline method uses multiple local polynomials to match the ground control points.
MAGNET provides multiple resampling options: bilinear, cubic, cubic spline, Lancoz and nearest neighbor. If a raster is categorical, nearest neighbor may more suitable. If the raster is continuous, cubic resampling method yields more smoother result.